Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field (B0 field) whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of an electromagnetic alternating field (RF field, also referred to as B1 field) of defined frequency (so-called Larmor frequency, or MR frequency). From a macroscopic point of view the distribution of the individual nuclear spins produces an overall magnetization which can be deflected out of the state of equilibrium by application of an electromagnetic pulse of appropriate frequency (RF pulse) while the magnetic field extends perpendicular to the z-axis, so that the magnetization performs a precessional motion about the z-axis. The precessional motion describes a surface of a cone whose angle of aperture is referred to as flip angle. The magnitude of the flip angle is dependent on the strength and the duration of the applied electromagnetic pulse. In the case of a so-called 90° pulse, the spins are deflected from the z axis to the transverse plane (flip angle 90°).
After termination of the RF pulse, the magnetization relaxes back to the original state of equilibrium, in which the magnetization in the z direction is built up again with a first time constant T1 (spin lattice or longitudinal relaxation time), and the magnetization in the direction perpendicular to the z direction relaxes with a second time constant T2 (spin-spin or transverse relaxation time). The variation of the magnetization can be detected by means of one or more receiving RF coils which are arranged and oriented within an examination volume of the MR device in such a manner that the variation of the magnetization is measured in the direction perpendicular to the z-axis. The decay of the transverse magnetization is accompanied, after application of, for example, a 90° pulse, by a transition of the nuclear spins (induced by local magnetic field inhomogeneities) from an ordered state with the same phase to a state in which all phase angles are uniformly distributed (dephasing). The dephasing can be compensated by means of a refocusing pulse (for example a 180° pulse). This produces an echo signal (spin echo) in the receiving coils.
In order to realize spatial resolution in the body, switched linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The signal picked up in the receiving coils then contains components of different frequencies which can be associated with different locations in the body. The MR signal data obtained via the RF coils corresponds to the spatial frequency domain and is called k-space data. The k-space data usually includes multiple lines acquired with different phase encoding. Each line is digitized by collecting a number of samples. A set of k-space data is converted to a MR image by means of Fourier transformation.
In a variety of MRI applications, motion of the examined object (the patient) can adversely affect image quality. Acquisition of sufficient MR signals for reconstruction of an image takes a finite period of time. Motion of the object to be imaged during that finite acquisition time typically results in motion artifacts in the reconstructed MR image. In conventional MR imaging approaches, the acquisition time can be reduced to a very small extent only, when a given resolution of the MR image is specified. In the case of medical MR imaging, motion artifacts can result for example from cardiac and respiratory cyclic motion, and other physiological processes, as well as from patient motion resulting in blurring, misregistration, deformation and ghosting artifacts.
Prospective motion correction techniques such as the so-called navigator technique have been developed to overcome problems with respect to motion by prospectively adjusting the imaging parameters, which define the location and orientation of the volume of interest within the imaging volume. In the image navigator technique hereby, a set of navigator signals is acquired at low-resolution from a spatially restricted volume, for example a navigator beam that crosses the diaphragm of the examined patient. For registering the navigator signals, so-called 2D RF pulses or 90 degree and 180 degree cross slab spin echo signals may be used. These excite the spatially restricted navigator volume in the form of a pencil beam, which is read out using a gradient echo. Other ways to detect the motion-induced momentary position of the volume of interest is the acquisition of two-dimensional sagittal slices that are positioned at the top of the diaphragm, or the acquisition of three-dimensional low-resolution data sets. The respective navigator volume is interactively placed in such a way that a displacement value indicating the instantaneous position of the anatomical feature to be imaged can be derived from the acquired navigator signals and used for motion correction of the volume of interest in real time. For example, the navigator technique is used for minimizing the effects of breathing motion in body and cardiac exams where respiratory motion can severely deteriorate the image quality. Gating as well as prospective and also retrospective motion compensation based on the navigator signals has been introduced to reduce these artifacts. Moreover, the navigator signals can be used to prospectively align several scans in an examination. After acquisition of the navigator signals subsequent imaging signals are acquired with the detected motion compensated for, reorientating the stack of image slices and collecting data during motion free time intervals. Finally a MR image is reconstructed from the acquired imaging signals.
As an example the navigator signal used in current coronary or renal MR angiography applications is typically the above-mentioned signal from a pencil beam shaped volume oriented through the diaphragm. Because the respiratory movements of the diaphragm and the heart and kidneys are correlated, the diaphragmatic navigator technique can be used to suppress respiratory motion artifacts in free-breathing coronary and renal MR angiography. However, the sensitivity and specificity of the diaphragmatic navigator approach in detecting stenoses of the coronary and renal arteries appears disappointing. One major factor is the diaphragmatic navigator itself which does not directly monitor the motion of the coronary and renal arteries. This consequently limits the effectiveness for suppressing motion artifacts.
Recently, navigator techniques are also used to prospectively detect and correct for head, prostate and joint motion. Therein coronal, sagittal and transverse localization slices or a 3D low resolution localization slab are acquired to detect translational and rotational motion. Since the afore-describes image navigator technique extends acquisition time, the detection of motion and respiratory states (like breath holds) may be achieved alternatively by external motion sensors (like optical and respiratory sensors) or k-space navigators (FNAV, ONAV, cloverleaf navigator). In case motion is detected or a respiratory state is reached, an image navigator is applied to prospectively adapt the imaging stack. Hence in general, a navigator can be applied in case motion is detected (e. g. head motion), a motion state is reached (e. g. end expiration breath hold) or in general interleaving navigators with the acquisition (e. g. free breathing motion).
The paper ‘Turboprop IDEAL: a motion resistant fat-water separation technique’ in MRM 61(2009)188-195 by D. Huo et al. discloses a motion resistant water-fat separation technique (TP-IDEAL). This known technique aims at avoiding motion artefcats in water-fat separation. This is achieved by averaging shift and rotation to avoid misregistration between the k-space blades in the Propeller acquisition.